Hard cheese …

A day or so after my return from my trip to Orleans, The Guardian published an article on deadly algae on Brittany’s beaches.   The alga in question is Ulva, which I have written about a few times in the past.  It is a genus that is often associated with elevated concentrations of nutrients (see “Venice’s green fringe” and “News from Qingdao …”).  In Qingdao, the accumulations of algae caused problems during the 2008 Olympic sailing events but, in Brittany, there have actually been deaths associated with these growths.   Although green algae do not produce toxins themselves, the mats are so thick that the algae at the bottom cannot get the light and oxygen they need and so die and rot.   However, the thickness of the mats also means that the bacteria involved in breaking down dead plant matter are also starved of oxygen and, under such conditions, they can use sulphate as an energy source.  This, however, produces the toxic gas hydrogen sulphide which accumulates until released by, in this case, people stepping on the mats.

The problem in Brittany is concentrated on the north coast, close to where the Seine empties into the English Channel, rather than the south coast, where the Loire joins the Bay of Biscay.  Seine or Loire, the problem is similar: France has a large and vociferous farming lobby and inorganic nutrients, much deriving from agriculture, spill out of the rivers into the sea where they encourage the growth of algae.  It is not just green algae: there are also toxin-producing dinoflagellate blooms which can render shellfish dangerous for human consumption.  The combination of seashores piled high with rotting algae and restaurants unable to source local produce for their “fruits de la mer” is a major worry in a region where tourism makes a significant contribution to the economy.   It is also the classic environmental challenge, as economically-rational activities have malign consequences 100 km or more away, creating major headaches for policy-makers.

There is, however, good evidence from modelling studies that a reduction in the nitrogen in rivers that empty into the coast around Brittany will have positive effects.  One of these went so far as to envisage the adoption of organic farming in all agricultural areas of the Seine basin, leading to a halving of nitrogen load and a likely very significant reduction in the frequency of dinoflagellate blooms.   Another study indicated a likelihood of much less Ulva if river nitrate concentrations were much reduced.

That’s the theory. Putting such reductions into practice is a different matter because it means taking on the farming lobby.  There is a simple logic, in a farmer’s eyes, to raising output by adding more of the nutrient that limits growth.  The flaw in the argument is that nitrate is highly soluble and a proportion of the nutrient that a farmer spreads on his fields will be washed into nearby water courses when it rains.   No farmer wants to pay for fertilizer that is not nourishing his plants so there ought to be a solution that is agreeable to both them and the environment.   In reality, implementing policies that protect one sector (seafood harvesting, in this case), whilst not undermining another (agriculture), all within a framework in which market forces drive much of the decision-making is a fiendish challenge.

I think that this is one of the reasons why right-leaning politicians are rarely enthusiastic about the environment: simply leaving market forces to decide outcomes means that “externalities” – consequences of a commercial activity that are not reflected in the price – will be ignored.   Environmental regulation implies a need for interventions to control activities in order to protect wider interests, but that is an anathema to free market purists.  Regulation should, in theory, limit the “externalities” and create an environment in which sectors such as agriculture, seafood harvesting and tourism can co-exist.  Again that’s the theory but regulating the environment invariably results in labyrinthine bureaucracies that soak up money from taxes which free market purists would prefer not to have levied in the first place.

That’s why I really would encourage you to read Kate Raworth’s Doughnut Economics (see “The limits of science …”).  Every environmental scientist needs to reflect on how the changes they want to see need structural alterations that permeate throughout society, and not just technological fixes.  And, yes, those changes might affect our own lifestyle too. If French farmers use less fertiliser then they will produce less milk per hectare.  That, in turn, will result in less of the wonderful French cheeses that we all love and, probably, higher prices.  So, in the final analysis, it is not just the use of nitrate fertiliser that will have to change, it is our own aspiration.  Before we can make a difference we will have to live differently ourselves.  That’s the tough challenge we all have to face.

References

Passy, P., Le Gendre, R., Garnier, J., Cugier, P., Callens, J., Paris, F., … Romero, E. (2016). Eutrophication modelling chain for improved management strategies to prevent algal blooms in the Bay of Seine. Marine Ecology Progress Series 543: 107-125.  https://doi.org/10.3354/meps11533

Perrot, T., Rossi, N., Ménesguen, A., & Dumas, F. (2014). Modelling green macroalgal blooms on the coasts of Brittany, France to enhance water quality management. Journal of Marine Systems 132: 38-53. https://doi.org/10.1016/j.jmarsys.2013.12.010

 

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Approaching the tipping point?

Erhai_hu_panorama_Apr19

From Kunming we travelled by train 200 km to the east, to Dàlī, which sits between Ērhāi Hú (literally ‘ear-shaped lake’), another of Yúnnán’s plateau lakes, and the Cāng Shān mountains.  These rise to about 4000 metres and still, even at this time of year, have patches of snow near their summits.   Dàlī’s old city has escaped the ravages of modernisation that have blighted many Chinese cities and we spent hours wandering the narrow streets lined with the traditional Bai architecture, along with a very large number of Chinese tourists.

On one day we hired bicycles and cycled along quiet roads lined with market gardens to reach the lake, then turned north and followed the lake shore for about five kilometres.  There were many areas of semi-natural shoreline along this stretch, with a fringe of wetland, but the filamentous algae (mostly Cladophora) coating the rocks that had been piled up along the settled parts of the shoreline told their own story.  This lake is clearly in better health than Diān Chí but it, too, is nutrient-rich.

Erhai_Hu_algae

A fringe of filamentous algae, along with floating leaves of Trapas natans(Eurasian water chestnut) growing on a jetty beside Ērhāi Hú.

Just after midday we pulled into a small family-run restaurant at the edge of a village.   A short conversation between the waitress and Ed (our only Mandarin-speaker) revealed that they specialised in serving fish from the lake so that seemed to be the obvious choice for our lunch.  The waitress disappeared, then reappeared with a net which she dunked into a tank behind our table and, with a couple of deft flicks, pulled it up with two carp wriggling inside.   These she quickly dispatched, cleaned and took into the kitchen.   About twenty minutes later she reappeared with a delicious stew comprising the whole fish cut into chunks, a generous seasoning of dried red chilli and tongue-numbing Sichuan pepper, lightly-pickled vegetables (white radish, celery and turnip) and some green leaves.   This, along with rice and endless green tea, and with Ērhāi Hú as a backdrop, created a perfect lunch for appetites whetted by our exercise.

Erhai_hu_fish_soup

From tank to table in twenty minutes: our fish stew made from carp from Ērhāi Hú.

We had seen some locals fishing with poles, and there is also some commercial fishing using cormorants in the area. However, the abundance of fish on the menu confirmed my hunch that the many booms we could see across bays along the lakeshore were fish farms (maybe ‘fish ranches’ is a more appropriate term when the fish have, relatively speaking, plenty of space in which to roam and forage).   The nutrients pouring into the lake here are, at least, able to support a economically-viable industry rather than undermine the supply of resources, as is the case for Diān Chí.

Of course, the story is not that simple.   In the past, Ērhāi Hú, like Diān Chí, had some endemic species, found nowhere else, but these have not been recorded for about 20 years.   Pollution from surrounding cities is the most likely explanation and, if Ērhāi Hú is in a better state than Diān Chí, then this is partly because the lake is larger and deeper, the catchment area is bigger and the scale of urban development is smaller (Dàlī is about a twentieth of the size of Kunming).  That said, local scientists have identified a significant declining trend in water quality, particularly over the past 25 years.   Importantly, however, they also note that it is not too late to do something about the situation.

Erhai_Hu_fishermen

Fishermen cleaning their nets beside Ērhāi Hú.  

In brief, nutrient concentrations in Ērhāi Hú are increasing. That leads to a more productive ecosystem which is, up to a point, good for commercial fishing but also means that oxygen concentrations drop, which is bad for the fish.   High nutrient concentrations also mean more algae but, at the present, these are not so high that cyanobacterial blooms develop as they have done in Diān Chí.  That means that the water in the lake can still be used as a drinking water supply for Dàlī and its environs.   However, if nutrient concentrations rise further then oxygen concentrations may pass a tipping point when it becomes almost impossible to manage lake phosphorus concentrations.

This is because phosphorus and other nutrients accumulate over time in lake sediments.  Phosphorus is not very soluble in the presence of oxygen, but becomes more soluble as conditions in the sediment and overlying water turn anoxic.   That means that when dissolved oxygen concentrations fall to the point where there is none at the sediment surface, the sediments are no longer a ‘sink’ for excess phosphorus, but become a ‘source’, releasing the stored nutrients back into the water.   From this point forwards, eutrophication in the lake becomes self-perpetuating and no amount of regulation alone will reverse this.

Better regulation now, on the other hand, might prevent the lake reaching this stage.  That, in turn, will protect the drinking water supply for the region, the economic benefits from the fishery and other ecosystem services. A survey of the local community revealed a willingness to pay an extra 27 Yuan a month for five years continuously in order to achieve this.  This is a small sum in absolute terms (27 Yuan is just over £3), but represents, on average, 1.7% of household income.   The economics of water quality improvement must look even more attractive to the regional government: if Ērhāi Hú crosses this tipping point then the investment in alternative water supplies, as was required in Kunming, will be equally expensive.  Looking at it from this perspective, applying a sensible ‘polluter pays’ policy now should be no more painful for the average resident than having to pay for new reservoirs to replace the resource on their doorstep.

The stretch of lakefront along which we cycled also had a steady trade in photographs, with photographers ready with diaphanous dresses for prospective models, and a number of ways for them to pose.  The girls in the photograph below posed, informally, on stones whilst friends photographed them using smartphones, but some photographers placed their models on the tops of jeeps or in hanging chairs, with an uninterrupted view of the lake behind. In their own way, they were valuing the broad scale panorama that the lake offered, just as we had enjoyed more local offerings during our lunch.  The challenge for the next decade, then, is to make the links between these valuations and the ecology of the lake, so that any price increases are recognised as sound investments in the future of Dàlī rather than as yet another form of negative taxation.

Erhai_hu_posers

Women posing for photographs with Ērhāi Hú as a backdrop

References

Wang, H., Shi, Y., Kim, Y. & Kamata, T. (2015).  Economic value of water quality improvement by one grade level in Erhai Lake: a willingness-to-pay survey and a benefit-transfer study.   Frontiers of Economics in China 10: 168-199.

Wang, S., Zhang, L., Ni, L., Zhao, H., Jiao, L., Yang, S., Guo, L. & Shen, J. (2015). Ecological degeneration of Erhai Lake and prevention measures.  Environmental Earth Sciences74: 3839-3847.

Zhang, K., Dong, X., Yang, X., Kattel, G., Zhao, Y. & Wang, R. (2018).  Ecological shift and resilience in China’s lake systems during the last two centuries.  Global and Planetary Change165: 147-159.

 

Blooms from above

Dianchi_lake_cyano_bloom

Saturday’s excursion saw us travelling to the southern end of the Kunming metro and joining a procession of locals trekking up the wooded slopes of the Xī Shān hills to the settlement of Lóng Mén (‘Dragon’s Gate’), which gave us some spectacular views over Diān Chí (Dian Lake) stretching away into the distance, After a lunch of fried noodles from one of the many takeaway stalls at Lóng Mén, we travelled back down to lake level by cable car, which gave us our second panoramic view of Cyanobacteria in three days.   The lake, China’s eighth largest, had a very conspicuous Cyanobacterial bloom that serves as the ‘yin’ to the Green Lake’s ‘yang’.

The environmental problems of Diān Chí are well known with an article in Newsweek describing it as the ‘ground zero of China’s toxic algae problem’.  The problems starts with Diān Chí’s location on a high plateau (1886 m above sea level) in Yunnan, which means that it has a relatively small catchment area relative to its size (40 km long, about 300 square kilometres area and with an average depth of 4.4 metres).   The city of Kunming sits at the north end of this lake and now has a population of over six million people.   For a long time, their untreated sewage was pumped directly into the lake, leading to high concentrations of phosphorus which, in turn, fertilised the lake water, allowing blooms of Microcystis aeruginosa to develop.   Many genera of Cyanobacteria, including Microcystis, produce potent toxins that attack the liver or nervous system, and which can cause skin rashes.

Unfortunately, the city of Kunming depended upon Diān Chí for its water supply in its past but now, due to this contamination, it has to rely upon reservoirs upstream of the city.  It has, according to the Newsweek article, invested $660 million dollars to reduce industrial pollutants, building sewage treatment works, intercepting polluted water and banning detergents containing phosphorus but that, as my photograph from the cable car shows, has had little effect.   There are two reasons for this.  The first of these is a reluctance to control fertiliser use in the productive agricultural areas to the west of the lake (China is not unique in this respect; a similar tardiness can be found in the West, where agriculture is a potent political lobby).  The second is that much of the phosphorus that was pumped into the lake in the past is still there, sitting in the sediments and being constantly recycled by the algae.  In small lakes it might be possible, albeit expensive, to dredge out this sediment but on a lake the size of Diān Chí this is an unimaginable prospect.

Another paper that I found online demonstrated a dramatic loss of higher plants and fish from Diān Chí. Since the 1950s, over half of all native higher plant species have been lost, along with 84 per cent of native fish.  Diān Chí also had a number of unique species, which evolved in this remote habitat, but 90 per cent of these, too, have been lost since the 1950s.  That is a catastrophe in biodiversity terms, but the collapse of the lake ecosystem also led to the loss of valuable commercial fisheries.  In the past, some of the fish and shellfish that we ate in local restaurants might have been bought from fishermen who worked the lake; now they have to be imported.

Dianchi_reaeration_equipment

A view from the cable car over Diān Chí, with yellow rafts bearing reaeration apparatus visible on the lake surface.  The picture at the top of the lake shows one edge of the Cyanobacteria bloom, with clearer water along a channel flushed by inflow from a lagoon.

We can see, in other words, another interesting case study in competing ecosystem services emerging. We might imagine a time in the far past when there was a balance between the use of the lake as a supply of resources (drinking water, fish and shellfish, irrigation water) was not compromised by the use of the lake’s natural biogeochemical cycles to break down any waste products that flowed in from the catchment.   More likely, human and animal wastes would have been recycled more directly as manure for local agriculture so, again, some sort of equilibrium would have pertained.   Now, we see the ‘provisioning’ services compromised due to the overuse of the ‘regulating’ services and, at the same time, opportunities for ‘cultural’ services such as recreation are also much reduced.

Thinking more widely, what about the ecosystem services lost due to the construction of the new water supply reservoirs around Kunming?   But then, rather than end on an overly sanctimonious tone, to what extent have we in the West, ‘solved’ some of our own environmental problems in recent decades through the contraction of our own manufacturing industries in the face of competition from countries such as China?  \

view_along_Dian_Chi

A view south along Diān Chí with the far shore, 40 km away, just visible in the distance.

References

Liu, J., Luo, X., Zhang, N. & Wu, Y. (2016).  Phosphorus released from sediment of Dianchi Lake and its effect on growth of Microcystis aeruginosaEnvironmental Science and Pollution Research23: 16321-16328.

Wang, S., Wang, J., Li, M., Du, F., Yang, Y., Lassoie, J.P. & Hassan, M.Z. (2013).  Six decades of changes in vascular hydrophyte and fish species in three plateau lakes in Yunnan, China.  Biodiversity and Conservation222: 3197-3221.

Zhu, L., Wu, Y., Song, L. & Gan, N. (2014).  Ecological dynamics of toxic Microcystis spp. and microcystin-degrading bacteria in Dianchi Lake, China.  Applied and Environmental Microbiology80: 1874-1881.

Notes:many authors, Western and Chinese, refer to ‘Dianchi Lake’.  However, as ‘chí’ means ‘lake’, I have just referred to ‘Diān Chí’ throughout.  See “Lake lakelake lake” for more about this. “La Grande Assiette de Lac Léman”  describes a similar conflict between ecosystem services in Lake Geneva, albeit with more positive outcomes.

 

Eutrophic or euphytic?

A paper has just been published that should be required reading for anyone interested in the management of nutrients in in ecology.   It is a follow-up of a 2006 paper with the catchy title “How green is my river” that set out to provide a conceptual framework for how rivers responded to enrichment by nutrients.   That original paper contained several good ideas but, crucially, not all of them were underpinned by evidence.  A decade on, several of the predictions and statements made in that original paper have been tested, and the time has come to re-examine and modify that original conceptual model.

My reaction to the 2006 paper was that it was very interesting but not fully reflective of the rivers in my part of Britain, whose rougher topography produced quite different responses to nutrient enrichment than that proposed in their original model.   That criticism has been addressed in the revised version, which places greater emphasis on the physical habitat template, which means that it is more broadly applicable than the original version.   But that, in turn, got me wondering about the continued relevance of a term such as “eutrophication” to rivers.

People have been using the term “eutrophic” to describe lakes with high concentrations of nutrients since early in the 20th century.   As the century progressed, evidence of a causal relationship between inorganic nutrients and algal biomass, and the consequences for other components of lake ecosystems grew.   With this foundation, it has then become possible to predict the benefits of reducing nutrients and there are plenty of case studies, particularly from deep lakes, that demonstrate real improvements as nutrient concentrations have declined.

Attempts to apply the same rationale to rivers have, however, met with far less success.   Legislation to reduce nutrients in rivers has been in force in Europe since 1991 (the Urban Wastewater Treatment Directive, followed by the Water Framework Directive) and whilst this has led to reductions in concentrations of phosphorus in rivers (see  “The state of things, part 2”), there has, in most cases, not been a corresponding improvement in ecology.   There are a number of reasons for this but, at the heart, there was a failure to understand that the tight coupling between nutrients and biology that was the case in deep lakes did not also occur in running waters.   What was needed was recognition of fundamental differences between lakes and rivers, and “How green is my river?” and, now, this new paper have both contributed to this.

However, one consequence of recognising the importance of the physical habitat template alongside nutrients is to challenge the relevance of the term “eutrophic” when describing rivers.   “Eutrophic” literally means “well-nourished” so is appropriate in situations where high nutrients cause high plant or algal biomass.   This high biomass (strictly speaking, the primary production arising from this biomass) then creates problems for the rest of the ecosystem (night-time anoxia caused by plants consuming oxygen being a good example).   If high biomass can arise due to, let’s say, removal of bankside shade or alteration to the flow regime, perhaps (but not always) in combination with nutrients, then perhaps we need a term that does not imply a naïve cause-effect relationship with a single pressure?

My suggestion is to shift the focus from nutrients to plant growth by using the term “euphytic” (“too many plants”) as this would shift the emphasis from simply driving down nutrient concentrations (expensive and not always successful) towards reducing secondary effects.  It is possible that strategies such as planting more bankside trees, for example, or altering the flow regime or channel morphology (see “An embarrassment of riches …”) may be just as beneficial, in some cases, as reducing nutrient concentrations.   That said, we also have to bear in mind that nutrients may have an effect well downstream, so focus on amelioration of effects within a particular stream segment will never be a complete solution.

I should emphasise that a lot of work has been done in recent years to understand the concentrations of nutrients that should be expected in undisturbed conditions, and also to understand the nutrient concentrations that lead to changes in community structure in both macrophytes and algae.   These show that many rivers around Europe do have elevated concentrations of nutrients and I am not trying to side-step these issues.  I do, however, think it is important that regulators can prioritise those rivers in greatest need of remediation and, in most cases, they do this without considering the risk of secondary effects.

It is, largely, a matter of semantics.   I have been involved in many conversations over the past couple of decades about how to improve the state of our rivers.  Many of those have centred on the importance of reducing nutrient concentrations (which would be, indisputably, a major step towards healthier rivers).  But there is more to it than that.  And Mattie O’Hare and colleagues are helping to open up some new vistas in this paper.

Note: the photograph at the top of this post shows the River Wear at Wolsingham.  This stretch of the river captures many of the challenges facing river ecologists: nutrient concentrations are relatively low and there is good bankside shade.  However, the flow of the river is highly altered due to impoundments upstream and a major water transfer scheme.  How do all these factors interact to create the often prolific algal growths that can be seen here, particularly in winter and spring?

References

Hilton, J., O’Hare, M., Bowes, M.J. & Jones, J.I. (2006).  How green is my river?  A new paradigm of eutrophication in rivers.   Science of the Total Environment 365: 66-83.

O’Hare, M.T., Baattrup-Pedersen, A., Baumgarte, I., Freeman, A., Gunn, I.D.M., Lázár, A.N., Wade, A.J. & Bowes, M.J. (2018).  Responses of aquatic plants to eutrophication in rivers: a revised conceptual model.   Frontiers in Plant Science.   9: 451

What does it all mean?

Just over a quarter of a century ago, my friend and colleague Steve Juggins and a group of other palaeoecologists came up with a clever way to relate the composition of diatom samples taken from different levels of a sediment core to the environmental conditions of the lake at the time that these diatoms were alive.   At the heart of this was a set of statistical tools called “transfer functions” and the use of these has proliferated over subsequent years, spilling from diatoms to many other groups of organisms and from palaeoecological studies to contemporary investigations of man’s impact on the environment.   So pervasive have these methods become that Steve returned to the subject a few years ago and critiqued the many misuses of the method that he was seeing in the literature.

The principle behind the use of transfer functions is that each species has a characteristic response to an environmental pressure gradient (in early studies this was pH) which could be portrayed as a unimodal (approximately bell-shaped curve).   The point along the gradient where a species is most abundant represents the “optimum” condition, the level of the pressure where the species thrives best.  The average of the optima of all organisms in a sample, Steve and colleagues showed, could be then used to estimate the value of the pressure.   This unlocked the door to quantitative reconstructions of changes in acidification of lakes in the UK and Scandinavia that, in turn, ultimately shaped environmental policy. It was one of the most impressive achievements of applied ecologists in the 20th century.

A diagrammatic representation of the principle behind transfer functions: each organism has a characteristic response to the predominant pressure (nutrient/organic pollution in this case).

Part of the reason for their success in building strong predictive models was, I suspect, that the pollutant that they were focussed upon had a direct effect on the physiology of the cells which, in turn, created strong selective pressures on the community.   Another reason was that palaeoecological samples condense all the habitat variation within a lake (plankton v benthic, seasonal differences etc) into a single assemblage.   This, then, begs the question of how well we should expect transfer functions to perform when applied to assemblages which represent much narrower windows of space and time, and when the pollutants of interest exert indirect rather than direct effects on the organisms.   Or, to recast that question another way, are some of the problems we encounter interpreting diatom indices from rivers another form of the misuse of transfer functions that Steve dissects in his review?

It is easy to believe that transfer functions do work when applied to contemporary diatom assemblages from rivers.   If you evaluate datasets you will almost certainly find that the “optima” for all the species do appear to be arranged along a continuum along the pressure gradient.  The question that we need to ask is whether this represents a causal relationship or is just a statistical artefact?  I touched on this issue in “What we expect is often what we get …” but, in that post, I was mostly interested in how samples react along a gradient, not the response of individual species.  I suspect that, given the importance of alkalinity in freshwater algal ecology (see “Ecology in the Hard Rock Café”), this must influence the distribution of optima along a nutrient gradient.   This will be compounded when sample sizes are small, as the likelihood is that the sample optimum will not correspond exactly to the “true” optimum for the species in question (a question Steve has also addressed in a more recent paper – see reference list below).  Finally, this is all embedded within a larger problem: that most of the work I have discussed here involves statistical inference from datasets compiled from samples collected from a range of sites in a region, but is intended to address changes in time rather than space (so-called “space-for-time substitution – see reference by Pickett below).   There has been relatively little testing of species preferences under controlled experimental conditions.

In practice, I suspect, the physiological response of benthic algae to nutrients is less complicated than our noisy graphs suggest.   I set out a version of this in “What we expect is often what we get …”.   That post dealt primarily with communities of microalgae; this is the same basic scheme (with some slight revisions) but posed in terms of the physiological response of the organisms.  It borrows from the habitat matrix conceptual model of Barry Biggs, Jan Stevenson and Rex Lowe (which, itself, builds on earlier work on terrestrial plants by Phil Grime and colleagues).

An alternative explanation for the response of benthic algae to nutrients and organic pollution.  a., b., c. and d. are explained in the text.

  1. Low nutrients / high oxygen concentrations – the “natural state” in most cases. Biggs et al. referred to species adapted to such conditions “stress-adapted” as they can cope in situations where nutrients are scarce. Associated with TDI scores 1 and 2.  Examples: Hannaea arcus, Achnanthidium minutissimum, Tabellaria flocculosa.
  2. high nutrients / no “secondary effects” of eutrophication – these are “competitive” species in Biggs et al.’s template and can thrive when there is anthropogenic enrichment of nutrients. Ideally, this group would consist of species that have a physiological adaptation that allows them to thrive when nutrients are plentiful though, in practice, our understanding is based mostly on inference from spatial patterns. The “window” where such species can thrive is wide, and will overlap with the two states described below, in many cases.  Associated with TDI scores 3 and 4.  Examples: Amphora pediculus, Rhoicosphenia abbreviata, Cocconeis pediculus.  Cladophora glomerata would be a good example of a non-diatom that belongs to this group.
  3. high nutrients plus “secondary effects” of eutrophication – this category extends the habitat template of Biggs et al. to include organisms whose are reacting to secondary effects  of nutrient enrichment (e.g. shade and low oxygen) rather than to the elevated nutrients per se and is, consequently, difficult to differentiate from a direct response to organic pollution. Associated with TDI scores 4 and 5. Examples include several species of Nitzschia as well as Mayamaea and Fistulifera, amongst others.   Importantly, this group may co-exist with representatives from group b. – perhaps inhabiting different zones of the biofilm that typically blend together when a sample is taken.
  4. high nutrients / very low oxygen – a final category that represents extreme situations when an ability to cope with reducing conditions is beneficial, and where diatoms that are facultative heterotrophs may thrive. Associated with TDI score 5. Heterotrophic fungal and bacterial growths (“sewage fungus”) may also be abundant.  Once again, there is likely to be some overlap between this and other groups.   Technically, this group is more likely to be associated with serious organic pollution than with nutrients; however, it will be found at sites where nutrient concentrations are high and it is possible that an association with nutrients may be inferred from spatial patterns.

We are left, in other words, with a choice between deriving optima along a continuous scale based on inferences from spatial patterns within which we know that there are significant confounding variables or dividing species into a few physiologically-defined categories for which there is not very much experimental underpinning.   Neither is ideal, and some of our recent analyses suggest that, in terms of model strength, there is little to choose between them.   The former, in my view, suggests an artificially high level of precision that is unrealistic, given the current state of knowledge.   The latter, on the other hand, links the data to a conceptual model rather than simply relying upon the numbers that squirt out at the far end of a statistical process.

That does not mean that such an approach might not be appropriate for some other groups of organisms.  The reason why I urge simplicity for diatoms is largely because of the scale of the habitats that we are sampling, in relation to the wider patterns of variability.  A continuous series of optima may be appropriate in some cases too.   Macrophytes surveys, for example, encompass all visible organisms found along a 100 m stretch.   These will have a range of life history and nutrient acquisition strategies: some of these will take up nutrients from the water, some from the sediments.  Different types of sediment will vary in the supply of phosphorus and nitrogen, and so on.   There will still be issues of confounding variables and risks of inferring from correlative rather than causal relationships, but perhaps the overall patchiness experienced over the survey length will create a more complex web of interactions between nutrients and community that justifies a continuous scale.

For diatoms, however, simplicity is probably the best choice.   In the absence of definitive evidence one way or the other we apply Occam’s Razor (“entities should not be multiplied unnecessarily”) and opt for the simpler of the two hypotheses pending evidence to the contrary.   This, in turn, may address a deeper issue: that of finding robust answers to complex problems (see “Unravelling causal thickets …”).   Inference from statistical models is only as good as the conceptual models that underpin those models and, I fear, we too often are so lost in the detail of the many confounding variables that we lose sight of our goals.  Being able to understand our observations in terms of ecological process is the first step to finding robust solutions to our problems.

References

Bennion, H., Juggins, S. & Anderson, N.J. (1996).  Predicting epilimnetic phosphorus concentrations using an improved diatom-based transfer function and its application to lake eutrophication management. Environmental Science & Technology 30: 2004-2007.

Biggs, B.J.F., Stevenson, R.J. & Lowe, R.L. (1991). A habitat matrix conceptual model for stream periphyton. Archiv für Hydrobiologie 143: 21-56.

Birks, H.J.B.,  Line, J.M., Juggins, S., Stevenson, A.C. & ter Braak, C.J.F.  (1990). Lake surface-water chemistry reconstructions from palaeolimnological data. Diatoms and pH reconstruction. Philosophical Transactions of the Royal Society of London Series B 327: 263-278.

Juggins, S. (2013).  Quantitative reconstructions in palaeolimnology: new paradigm or sick science?  Quaternary Science Reviews 64: 20-32.

Kelly, M.G., King, L. & Ní Chatháin, B. (2009).  The conceptual basis of ecological status assessments using diatoms.  Biology and Environment: Proceedings of the Royal Irish Academy 109B: 175-189.

Pickett, S.T.A. (1988).  Space-for-time substitution as an alternative to long-term studies.  Pp. 110-135.   In: Long-term Studies in Ecology: Approaches and Alternatives (edited by G.E.. Likens).  Springer-Verlag, New York.

Reavie, E.D. & Juggins, S. (2011).  Exploration of sample size and diatom-based indicator performance in three North American phosphorus training sets.  Aquatic Ecology 45: 529-538.

The challenging ecology of a freshwater diatom?

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Amphora pediculus from Polly Brook, Devon, December 2016. Scale bar: 10 micrometres (= 1/100th of a millimetre).

The images above show one of the commonest diatoms that I find in UK waters.  It is a tiny organism, often less than 1/100th of a millimetre long, which means that it tests the limits of the camera on my microscope.  In recent months, however, it is not just the details on Amphora pediculus’ cell wall that I am struggling to resolve: I also find myself wondering how well we really understand its ecology.

The received wisdom is that Amphora pediculus favours hard water, does not like organic pollution and is relatively tolerant of elevated concentrations of inorganic nutrients.  This made it a very useful indicator species in a period of my career when we were using diatoms to identify sewage work s where investment in nutrient-removal technology might yield ecological benefits.  There were many nutrient-rich rivers, particularly in the lowlands, where any sample scraped from the upper surface of a stone was dominated by these tiny orange-segment-shaped diatom valves.   Unfortunately, twenty years on, many of those same rivers have much lower concentrations of nutrients (see “The state of things, part 2”) but still have plenty of Amphora pediculus.   Did I get the ecology of this species wrong?

The graph below shows some data from the early- and mid- 1990s showing how the abundance of Amphora pediculus was related to phosphorus.   The vertical lines on this graph show the average position of the boundaries between phosphorus classes based on current UK standards.   Records for A. pediculus are clustered in the “moderate” and “poor” classes, supporting my initial assertion that this species is a good indicator of nutrient-enriched conditions, but there are also samples outside this range where it is also abundant, so A. pediculus is only really useful when it is one of a number of strands of evidence.

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The relationship between Amphora pediculus and reactive phosphorus in UK rivers, based on data collected in the early-mid 1990s.  Vertical lines show the average boundaries between high and good (blue), good and moderate (green), moderate and poor (orange) and poor and bad (red) status classes based on current UK standards and the two arrows show the optima based on this dataset (right) and data collected in the mid-2000s (left).

If we weight each phosphorus measurement in the dataset by the proportion of Amphora pediculus at the same site (i.e. so that sites where A. pediculus is abundant are given greater weight), we get an idea of the point on the phosphorus gradient where A. pediculus is most abundant.   We can then infer that this is the point at which conditions are most suitable for the species to thrive.  In ecologist’s shorthand, this is called the “optimum” and, based on these data, we can conclude that the optimum for A. pediculus is 154 ug L-1 phosphorus.  The right hand arrow indicates this point on the graph below. However, I then repeated this exercise using another, larger, dataset, collected in the mid-2000s.   This yielded an optimum of 57 ug L-1 phosphorus (the left hand arrow on the graph), less than half of that suggested by the 1990s dataset.   There are, I think, two possible explanations:

First, the 1990s phosphorus gradient was based on single phosphorus samples collected at the same time that the diatom sample was collected (mostly spring, summer and autumn) whilst the mid-2000s phosphorus gradient was based (mostly) on the average of 12 monthly samples.  As phosphorus concentrations, particularly in lowland rivers, tend to be higher in summer than at other times of the year, it is possible that part of the difference between the two arrows is a result of different approaches.  (For context, in the 1990s, when I first started looking at the effect of nutrients in rivers, phosphorus was not routinely measured in many rivers, so we had no option but to do the analyses ourselves, and certainly did not have the budget or time to collect monthly samples).

However, another possibility is that the widespread introduction of phosphorus stripping in lowland rivers in the period between the mid-1990s and mid-2000s means that the average concentration of phosphorus in the rivers where conditions favour Amphora pediculus have fallen.   In other words, A. pediculus is tolerant of high nutrient conditions but is not that bothered about the actual concentration.   My guess is that it thrives under nutrient-rich conditions so long as the water is well-oxygenated and, as biochemical oxygen demand is generally falling, and dissolved oxygen concentrations rising (see “The state of things, part 1”), this criterion, too is widely fulfilled.   I suspect that both factors probably contribute to the change in optima.

But the second point in particular raises a different challenge:  We often slip into casual use of language that implies a causal relationship between a pressure such as phosphorus and biological variables whereas, in truth, we are looking at correlations between two variables.   Causal relationships are, in any case, quite hard to establish and the effect that we call “eutrophication” is really the result of interactions between a number of factors acting on the biology.   All of these simplifications mean that it is useful, from time to time, to look back to see if assumptions made in the past still hold.   In this case, I suspect that some of our indices might need a little fine-tuning.  There is no disgrace in this: the evidence we had in the 1990s led us to both to a conclusion about the relative sensitivity of Amphora pediculus to nutrients but also fed into a large-scale “natural experiment” in which nutrient levels in UK rivers were steadily reduced.   When we evaluate the results of that natural experiment we see we need to adjust our hypotheses.  That’s the nature of science.  As the sign on the door of a friend who is a parasitologist reads: “if we knew what we were doing, it wouldn’t be research”.

References

The 1990s dataset (89 records) is mostly based on data used in:

Kelly M.G. & Whitton B.A. (1995).   A new diatom index for monitoring eutrophication in rivers.   Journal of Applied Phycology 7: 433-444.

The mid-2000s dataset (1145 records) comes from:

Kelly, M.G., Juggins, S., Guthrie, R., Pritchard, S., Jamieson, B.J., Rippey, B, Hirst, H & Yallop, M.L. (2008).   Assessment of ecological status in UK rivers using diatoms.   Freshwater Biology 53: 403-422.

The camera never lies?

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The picture above shows a rather unprepossessing view of a river bed, photographed earlier this month.   The stones, to give a sense of scale, are all less than ten centimetres across.    What is your immediate reaction?   My guess is that it is probably negative: that mass of green filaments cannot indicate a healthy ecosystem.   However, the next picture is a view of the same river bed photographed a month earlier and that shows a very different scene.  There are just a few tufts of filamentous algae, if you look closely but, overall, the stones are clean.   First impressions, at least, are not negative.

That phrase “first impressions” is important.   If you were to take a closer look at the composition of the plants and animals at this site, you will see little to cause concern.  There is a good diversity of algae and invertebrates, and these include several that thrive only in high quality rivers.   The larger plants, too, are those that we associate with rivers with low nutrient concentrations and there are also salmon and trout present.   There are issues with the river but these are not my primary concern today.  What is of interest to me today is the reason behind the negative reaction.

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The same river bed as the upper photograph, but photographed in August 2016, rather than September 2016.  Both photographs taken on an Olympus  TG2 camera.

There is a trend for pictures such as the one above to be included in reports.  The reason is, I think, quite straightforward: waterproof digital cameras of a reasonable quality are now sufficiently affordable that many of us carry them about as standard parts of our field kit.  They are useful for documenting many different aspects of the aquatic world but I worry that the audiences for these pictures have few opportunities to calibrate their experiences.

The contrast between the two pictures illustrates the danger of relying on a single photograph to infer the condition of a stream or lake.   Many types of aquatic survey may take place annually; a picture in a report can, therefore, never be wholly representative of the state of algae at a site, as quantities can change rapidly.   Inferring the condition of a water body from a short-lived fast-responding group of organisms is never straightforward and depends upon those interpreting the data (and, in this instance, visual evidence) being able to place this into context.  I worry when I see pictures such as those above included in reports of surveys of aquatic plants, in particular, because surveyors are used to studying organisms with longer life-cycles and more stable assemblages.   A photograph of mass algal growths offers a “snapshot” with few guarantees that this is typical for the the lake or stream under consideration.   The reality is that the beds of even healthy streams turn green for brief periods during the year; the problem for the surveyor unversed in algal lore, is how to separate “signal” from “noise”.

Some of my earlier posts have demonstrated the advantages that a close-up view of the underwater world that these cameras offer to freshwater biologists (see “Bollihope Burn in close-up”).   We are in a better place through having the ability to record the underwater world directly, rather than simply naming, counting and measuring; photography gives us a higher level cognitive experience and a more holistic overview of systems.  But these rewards are accompanied by new challenges.   In the same way that Wikipedia is an asset, only if used with safeguards to ensure that information that is presented can be verified; therefore we need to treat photographs of the underwater world with respect.   As for most of our technological advances, they complement, rather than replace, existing knowledge and wisdom.